1. Field of the Invention
The present invention relates to an inductively coupled reception coil arrangement for magnetic resonance tomography having at least two individual coils, including an outer primary coil serving as the output coil for an inner secondary coil.
2. Description of the Prior Art
In magnetic resonance tomography, local coils are employed for imaging different body parts, the local coils being specifically adapted by means of their geometry and their receiving profile, since it is important to have a high sensitivity factor of the coil for the purpose of achieving a high signal-to-noise ratio. This means that the volume of the coil and the sample should be the same size as far as possible. Consequently, use is made in practice of coils of different sizes for head, knee and wrist.
It is also known to introduce a second, smaller coil into a larger coil, the two coils being coupled inductively, since it is possible in this way to improve the sensitivity factor.
New oscillation modes arise, however, in the case of inter-nesting of two coils. The coils influence one another mutually such that the receiving coil must be re-tuned in each case, or the receiving coil is used only as output coil and does not itself contribute to imaging. Specifically in the case of circular coils, re-tuning is very complicated because of the additional coupling of the linear systems.
It is an object of the present invention to configure an inductively coupled reception coil arrangement of the type initially described so as to avoid undesired mutual influencing of the coils, and thus to avoid the need for re-tuning.
This object is achieved in accordance with the invention by providing the primary coil with a pre-amplifier decoupling device which closes the coil with high resistance. In an embodiment the pre-amplifier decoupling device has an input impedance of the pre-amplifier such that, together with an interface circuit, it constitutes a parallel resonance point at the low end of the coil.
Such a coil with pre-amplifier decoupling no longer has a resonance point, and so the inductively coupled coil (secondary coil) remains at resonance when introduced into the output coil (primary coil), there being only a slight influence on the unloaded Q of the secondary coil. This is greatly advantageous in the case of a circular design, in particular.
A further advantage of the design according to the invention with a high-resistance closure of the primary coil via a pre-amplified decoupling is that the primary coil can be used both as an imaging receiving coil alone or as output coil for the secondary coil.
It is possible to dispense entirely with load-dependent re-tuning by means of a low-noise design of the pre-amplifier.